The present invention concerns thin-laminate panels used to form capacitive printed-circuit boards, and more particularly copper-clad thin-laminate panels and methods for making the same.
Conductor plates (xe2x80x9clayersxe2x80x9d) or printed circuit boards (PCBs) are typically made of one or more panels of insulating or dielectric material having a continuous thin-layer of a conductive material, such as copper, laminated one or both sides thereof. A desired layout or pattern of conducting paths and connection areas of printed circuits is then made by removing selected regions of the conductive layer from the dielectric material, to produce a PCB. One or more devices (typically integrated circuits) are then formed on or mounted on the PCB.
More specifically, each panel of the PCB typically includes a dielectric material, such as resin-impregnated fiberglass cloth layer (xe2x80x9cdielectric layerxe2x80x9d). The panel further includes thin conductive layers (e.g., copper foil) laminated to each side of the dielectric layer. The thin-laminate panels provide necessary capacitance for all or a substantial number of the integrated circuits formed on the xe2x80x9ccapacitivexe2x80x9d PCB.
Conventional PCBs are made of multiple, thick panels (termed xe2x80x9cthick laminatesxe2x80x9d) having a thickness of about 0.059 inches or more (i.e., about 1.5 mm). However, electrical engineers are designing integrated circuits that require thinner and thinner dielectric spacing between the conductive layers that form the printed circuits. Accordingly, the thickness of the dielectric material in such panels (and thus the distance between the conductive layers) has become relatively small, typically about 0.006 inches (i.e., about 1.5 mm) or less. The actual thickness of a particular dielectric layer of a thin-laminate panel is determined based on the level of capacitance necessary for the particular integrated circuits to be formed on the capacitive PCB made from the panel. Such laminate panels are termed xe2x80x9cthin laminatesxe2x80x9d or xe2x80x9cthin-laminate panels.xe2x80x9d
The failure rate of the thin-laminate panels used to make capacitive PCBs is often high due to manufacturing defects in the thin-laminate panels themselves. The thin-laminate panels are often defective due to various factors, such as impurities in the material comprising the dielectric layers of the panels. Conductive material introduced into the dielectric layers typically causes shorts between the conductive layers forming the circuit layers (defined by the selective removal of the conductive layers). Thus, the panels would be preferably tested for such manufacturing defects prior to processing the panels to produce the capacitive PCBs. Unfortunately, the prior art thin-laminate panels cannot be accurately tested prior to processing of the PCBs due to the presence of conductive material on the edges of the dielectric layers of the panels.
Specifically, thin-laminate panels are initially made in large sheets and the sheets are then sheared into relatively small panels from which PCBs are made. With reference to the prior art thin-laminate panel illustrated in FIG. 1(a), as the large sheets of thin laminate are cut or sheared to provide multiple thin-laminate panels, the relatively soft conductive layer 6 of the laminate typically smears and spreads conductive material 6a across one or more edges 12 of the panel. The connection of the two conductive layers 6 formed via the smeared conductive material 6a makes it difficult to accurately test such panels for electrical deficiencies.
More particularly, the panels would preferably be tested for undesirable electrical connection between the two conductive layers of the panel due to the presence of conductive material within the dielectric layers. Such an electrical connection through the dielectric layer will not be removed with subsequent processing of the PCB. Thus, the connection through the dielectric layer may result in a defective PCB. When testing the panels for such impurities, electrical connection between the two conductive layers due to conductive material on the edge(s) of the dielectric layer, results in false failures. That is, the conductive material on the panel edges make it appear as though there is electrical conductance through the dielectric layer. Put another way, the smeared conductive material on the dielectric layer edge(s) results in short circuits between the conductive layers of the panel, signifying that the panel is defective due to electrical conductance through the dielectric layer, even though the thin-laminate panel may not be defective. The short circuits at the edges of the panels may be avoided by subsequent PCB processing of the panels. However, because conductive material on the edges of the thin-laminate panels cause short circuits that provide false failures, the panels are not tested before processing the panels to produce the PCBs. That is, if the panels are tested before processing the PCB, otherwise non-defective thin-laminate panels are discarded as being defective due to false failures caused by the smeared conductive material. PCB processing of the panels is both expensive and time consuming as are the laminate panels themselves. Thus, effective and accurate testing of the thin-laminate panels before processing the PCBs and the avoidance of false failures provide distinct advantages.
Referring to the prior art thin-laminate panel illustrated in FIG. 1(b), cutting of the panel need not result in conductive material 6a being smeared completely across an edge 12 of the dielectric layer 8 to cause problems. That is, smeared conductive material that only partially covers an edge 12 of a dielectric layer 8 reduces the already small distance between the conductive layers 6 of the panel 2. This reduced distance between the conductive layers makes it difficult to accurately test such panels for electrical deficiencies. When testing the panels, the reduced distance typically allows conduction of electricity from one conductive layer to the other, making it appear as though there is electrical conductance through the dielectric layer. Such panels are then discarded as defective because it is impossible to determine whether the xe2x80x9cshort circuitxe2x80x9d is due to conductive impurities in the dielectric layer or is simply due to conductive material on the edges of the dielectric layer (which would be eliminated during processing of the PCBs). Accordingly, with thin-laminate panels even a minor amount of conductive material on the edge of the dielectric layer may be detrimental.
Because conventional PCBs are made from thick-laminate panels (e.g., about 0.059 inches (about 1.5 mm)), conductive impurities in the dielectric material is typically not a problem due to the relatively thick layer of dielectric material. In other words, the relatively large mass of dielectric material of the dielectric layer typically limits any deleterious affects of such conductive impurities. As a result, conductive material on the edges of the dielectric layer of thick-laminate panels typically is not a problem, as the thick-laminate panels need not be tested.
In the past, bevelers have been used to give the edges of thick-laminate panel PCBs a rounded profile. The rounded profile makes the thick-laminate panel PCBs safer to handle. Such beveling does not (and was not intended to) produce any electrical benefits. That is, once the laminate panels are processed to define the PCBs, there is not any conductive material located at regions of the laminates that would result in smearing. Additionally, current beveling processes require that the thick-laminate PCBs be beveled one at a time.
Accordingly, there is a need for thin-laminate panels for the production of PCBs, wherein the thin-laminate panels have edges that have been finished so that the edges are free of conductive material. The absence of conductive material on the edges of the dielectric layers of the panels allows testing of the panels for manufacturing defects prior to processing of the PCBs and avoids discarding non-defective thin-laminate panels due to false failures. Further, there is a need for methods for finishing edges of thin-laminate panels in a manner that does not cause smearing of conductive material along the edges of the dielectric layers and apparatus for performing the same.
In light of the deficiencies of the prior art, the present invention provides xe2x80x9cfinishedxe2x80x9d thin-laminate panels having dielectric layers of about 0.006 inches or less. Thin conductive layers (e.g., copper) are laminated on either side of the dielectric layers and the edges of the dielectric layers of the thin-laminate panels are finished to be free of conductive material. As discussed above, the thin-laminate panels are designed to provide necessary capacitance for all or a substantial number of integrated circuits to be mounted on or formed on a PCB made from one or more of the panels. The finished thin-laminate panels of the present invention may be tested for manufacturing defects, such as short circuits, before further processing of the panels to produce PCBs. Further, such testing may be done without the risk of false failures resulting in discarding of non-defective panels, because the edges of the dielectric layers of the panels of the present invention are free of conductive material.
The present invention further provides xe2x80x9cfinishingxe2x80x9d methods for removing conductive material from the edges of the unfinished thin-laminate panels in a manner that does not cause further smearing of the conductive material onto the dielectric layer. Preferably a CNC router having a vertically mounted router bit is used. To assure that conductive material from the conductive layers of an unfinished thin-laminate panel is not smeared onto the edges of the dielectric layer as a portion of the edge of the panel is removed, the rotation of the router bit is made to coincide with the plane defined by the surface of the panel. Additionally, the router bit has cutting edges that extend axially rather than spirally or helically, so that the cutting action is in the direction of the router bit rotation. Further, the router bit is not moved vertically relative to the plane of the panel at any point during the shearing process.
In an alternative embodiment of the present invention, thin-laminate panels are cut from a thin-laminate sheet using a circular saw blade. To significantly reduce smearing of conductive material onto one or more edges of the dielectric layer of the panel being cut from the sheet, a circular saw blade having a specified number of teeth is utilized. Further, the blade is rotated within a specific RPM range, and is operated to cut the sheet into panels within a specific range of speed. Such saw blade xe2x80x9cparamentersxe2x80x9d substantially prevent smearing of conductive material from the conductive layers of the sheet onto the edges of the dielectric layer as the panel is cut from the sheet. If necessary, the panel edges are then xe2x80x9cfinishedxe2x80x9d by removing small portions of one or more of the edges of the panel using an abrasive material, such as sandpaper.
The present invention further provides securing apparatus or fixtures for securing one or more unfinished thin-laminate panels to a surface for removal of conductive material from the dielectric layer edges. The thin-laminate panels are mounted in the securing apparatus to shear the edges of the panels and produce the finished thin-laminate panels of the present invention. Such securing apparatus of the present invention allows the panels to be secured to a surface during the shearing process without the need to drill holes through or otherwise diminish useful portions of the panels.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of preferred embodiments that proceed with reference to the accompanying drawings.